1. Field of the Invention
The present invention relates to a hydraulic brake pressure controlling apparatus for a vehicle, which is employed during an anti-lock control operation preventing a lock of vehicle wheels when the vehicle brakes, or during a traction control operation preventing a slippage of driving wheels of the vehicle when the vehicle starts to move and/or accelerates.
2. Description of the Related Art
A known anti-lock control apparatus is intended for preventing the vehicle wheels from locking when the vehicle brakes and for achieving a running stability and shortening the brake distance. In the anti-lock control apparatus, it is provided a microcomputer which determine a control mode (pressure increasing mode, pressure decreasing mode and holding mode) of the brake fluid pressure in accordance with a signal representing a speed of the vehicle wheel detected by a vehicle wheel speed sensor, and increasing, holding or holding the brake fluid pressure by opening or closing hold valves (pressure increasing and holding valve) and decay valves (pressure decreasing valve) formed of a solenoid valve, corresponding to one of the control modes.
There has been known a conventional hydraulic brake pressure controlling apparatus in which a fluid pressure control member (modulator) for the anti-lock control operation described above is provided together as one unit with a master cylinder. In such an apparatus, as disclosed in U.S. Pat. No. 4,415,210, for example, a master cylinder is provided with a primary piston and secondary piston arranged in series with each other to form a tandem type. These pistons control fluid pressure of two brake device systems, respectively. In the apparatus, further, hold valves of normally-open type are disposed in fluid passages communicating between two fluid pressure chambers actually controlled by the primary and secondary pistons and respective wheel cylinders, and decay valves of normally-closed type are disposed in a passage communicating between the wheel cylinders and a reservoir.
When a brake pedal is depressed down to thereby supply the fluid pressure in the fluid pressure chamber of the master cylinder to the wheel cylinders through hold valves and, accordingly, the fluid pressure in the wheel cylinder increases to apply a braking force to the vehicle wheels. After that, when the anti-lock control operation starts, the hold valves close to hold the brake fluid pressure. In this condition, when the decay valves open while closing the hold valves, the brake fluid in the wheel cylinders is discharged to the reservoir through the decay valves, so that the fluid pressure in the wheel cylinder is decreased to thereby decreasing the braking force. On the other hand, during the pressure increasing mode in the anti-lock control operation, the hold valves open while the highly-pressurized brake fluid supplied from the fluid pressure source such as an accumulator is applied to the pistons, to thereby increase the fluid pressure in the wheel cylinders.
However, if in the case that the fluid pressure source system such as a fluid pump or the like is failed during the anti-lock control operation, since the braking force decreases, the brake pedal must be depressed down more strongly. Such is disadvantageous in that it is difficult to maintain the sufficient remained stroke of the brake pedal.
In view of the foregoing difficulty accompanying the conventional apparatus, the present inventor has proposed, as disclosed in commonly assigned U.S. Pat. No. 4,867,509, an improved hydraulic brake pressure controlling apparatus capable of eliminating the difficulty. In this apparatus, it is provided a fluid passage communicating the fluid pressure chamber of the master cylinder with an accumulator, and in the fluid passage a supply valve of normally-closed type which opens merely during the anti-lock control operation. The apparatus is further provided with intake valves in opening parts of the fluid passage which parts communicating with the fluid pressure chamber, so that head part of the intake valves are protrudable into the fluid pressure chamber thereby shutting the fluid passage. Furthermore, the apparatus is provided with valve operating members movable together with the pistons of the master cylinder for opening the intake valves in such a manner that the valve operating members engage with end portions of the intake valve when the stroke of the piston reaches a predetermined amount.
In the hydraulic brake pressure control apparatus, since the valve operating members open the intake valves in accordance with the movement of the pistons of the master cylinder, during the anti-lock or traction control operations in which the supply valve opens, the highly pressurized brake fluid is supplied from the accumulator into the fluid pressure chamber of the master cylinder, so that the piston is pushed back to a predetermined position, thereby maintaining a sufficient stroke of the brake pedal in case of failing the fluid pressure source system and, therefore, a required braking force in proportion to the depressing force applied to the brake pedal can be supplied to the wheel cylinders to brake the vehicle.
However, such an apparatus is still disadvantageous in that the apparatus requires the accumulator having a large capacity and a pressure switch for supervising the fluid pressure in view of the fail-safe, which cause the apparatus to be large in size, heavy in weight, and high in manufacturing cost.